1. Field of the Disclosure
The disclosure is generally related to coated colloidal materials, and more specifically is related to silica-coated transition metal nanocrystals, such as silica-coated silver or gold nanoprisms.
2. Brief Description of Related Technology
Metal nanoparticles have attracted a great deal of attention during the past decades due to their potential applications in the fields of catalysis, optics, and biosensing. Gold and silver nanostructures are of particular interest due to their unusual optical properties that are dependent on size and shape. Triangular silver nanoprisms, in particular, exhibit highly tunable architecture-dependent optical properties. These structures also have very high surface energies, especially at their tips and edges, where the silver atoms can be readily oxidized. Unfortunately, this oxidation causes either truncation of the tips of the prisms or their complete dissolution and is accompanied by a concomitant shift in or a complete loss-of-their surface plasmon resonance (SPR) bands. Consequently, methods have been investigated in the attempt to protect nanoparticles from oxidation.
One method that has been attempted to protect nanoparticles from oxidation is encasing the nanoparticles in silica shells. These shells are useful because they 1) are transparent in the visible and IR regions of the spectrum, 2) are chemically inert in a wide variety of solvents, and 3) can be functionalized using well-developed silane coupling chemistry. Therefore core-shell nanostructures would typically maintain the optical signatures of the metal cores, while gaining the desirable chemical and physical properties of the silica shells.
The Stöber method recently has been adapted for coating metal nanocrystals with silica shells. See e.g. Alsan et al. J. Am. Chem. Soc. 129:1524 (2007). This sol-gel process typically involves ammonia-catalyzed hydrolysis and condensation of molecules, such as tetraethoxysilane. However, directly applying this method to coat silver nanoparticles poses challenges because etching and aggregation of silver nanoparticles are induced by ammonia.
To solve this problem, Kobayashi et al. have used dimethylamine (DMA) (at concentrations between 0.4 and 0.8 M) to catalyze silica shell growth on silver nanoparticles. See Kobayashi et al. J. Colloid Interf. Sci. 282:392 (2005). Using DMA as opposed to ammonia, the researchers showed that silica shells could be easily formed without decreasing the diameter of the silver nanoparticle core. However, Kobayashi's method cannot be applied to silver nanoprisms because significant etching and aggregation of the nanoprisms occur even in a 0.4 M solution of DMA.
Therefore, a need exists for the preparation of coated nanoparticles where the size and shape of the nanoparticle is maintained through the coating process.